We are entering an era of transition in mobile wireless networks from second generation systems (2G systems) to third generations systems (3G systems). There is a large installed base of 2G systems throughout the world, including GSM (Global System for Mobile Communication). 3G systems include an array of proposed standards, two of which include CDMA2000 and UMTS (Universal Mobile Telecommunications System). 3G systems offer a host of advantages over 2G systems, at least in part because 3G systems are designed to handle the every increasing variety of traffic types that user's access over the air. Whereas 2G systems were designed with voice traffic as the primary type of traffic, 3G systems were designed with data traffic as a primary traffic type.
This evolution of wireless thought can best be seen by the data rates at which 2G and 3G systems typically operate. Current GSM can typically deliver speeds of 10–50 kbits/second and utilize TDMA (time division multiplex access) over channels of, for example, 200 kHz in bandwidth. 3G systems are designed to deliver much higher data transmission speeds over correspondingly wider bandwidths. For example, UMTS may deliver at data transmission speeds over 2 Mbits/second utilizing CDMA (code division multiple access) on channels of 5 MHz bandwidth.
A UMTS may be broken up into two portions: the core network and the radio network. The radio network of a UMTS is typically called the UMTS Terrestrial Radio Access Network (UTRAN) system. In UTRAN, communication may take place between a handset or remote terminal(more generally referred to as an example of user equipment or UE) and a base station (NodeB) utilizing a Frequency Division Duplex (FDD) mode communicating using CDMA. In FDD mode, separate channels are used for uplink and downlink of information. As defined by UMTS standards, transmission of data occurs within a 10 ms radio frame. The radio frame is divided into 15 slots, with each slot consisting of 2560 chips per slot. Multiplying the number of chips per slot by the number of slots within the 10 ms. radio frame yields a chip rate of 3.84 Mchips/second.
A “chip” is a unit of information transmitted over a spread spectrum system (such as CDMA) after a spreading code has been applied to the incoming bit stream. In order to spread an incoming bit stream, the incoming bit stream is multiplied by a spreading code having a greater bit rate, with the greater bit rate being based on the spreading factor. The spreading code in the UMTS standard is actually comprised of two different codes: a channelization code and a scrambling code. The incoming bit stream is first multiplied by the channelization code, with the resulting product multiplied by the scrambling code. The channelization code, also known as Walsh code, is used in the downlink direction to uniquely identify the UE to which the data is being delivered. While in the uplink direction, the channelization code is used to distinguish data and control channels from the same UE. The scrambling code or spreading code is used in the downlink direction to separate cells, and in the uplink direction, the scrambling code is used to identify the UE from which the signal originates.
In UMTS, scrambling codes are typically selected to be of the same length as a frame, e.g., 38,400 chips in UTRAN, and are selected from an even longer pseudo-random sequence. Channelization codes are variable length codes, where the code length is based on the spreading factor selected for the transmission. Spreading factors vary from 4 to 512 in the downlink direction and from 4 to 256 in the uplink direction. Since the length of the channelization code is based on the spreading factor, the number of chips produced for each incoming bit is directly related to the spreading factor. For instance, a UTRAN system using a spreading factor of 4 would produce 4 chips per incoming bit. While the spreading factor may vary, the chip rate remains at 3.84 Mchips/second in the UTRAN standard. A more detailed description of UMTS networks can be found in UMTS Networks: Architecture, Mobility and Services, by Heikki Kaaranen, et al., John Wiley & Sons, 2001.
Clearly, it is a daunting task for a network provider to migrate from existing 2G systems to 3G systems, such as UMTS. During this period of change, mobile network providers are faced with the problem of migrating their networks and users from 2G systems to 3G systems. These problems include, for example, allocated spectrum bandwidth constraints and high startup costs to rapidly transition from 2G systems to 3G system.
For example, current network providers often have limited allocated spectrum bandwidth from the government. It is typical for a network provider to only be allocated 30, 20, or as little as 10 MHz in a particular market. When migrating from a 2G system to a 3G system, this lack of available bandwidth may be a significant constraint. It is unlikely that a network provider can set aside the 10 MHz necessary (5 MHz transmit frequency+5 MHz receive frequency) to deploy a single channel in a UMTS without partially abandoning their installed base of clients due to the tremendous band amount of bandwidth currently required to startup a 3G system, such as UMTS. In a sense, this is a granularity or step-size problem. A network provider can not gradually step into a 3G system as currently specified, but must jump in with both feet.
Tremendous initial startup costs also plague the transition from 2G to 3G systems. Because of the bandwidth requirements of 3G systems, such as UMTS, the 3G systems will crowd out the 2G systems from the allocated spectrum bandwidth of a network provider. This forces the network provider to make a wholesale switch from 2G to 3G networks, which entails tremendous initial startup costs to the network provider.
The present invention is directed to overcoming the one or more problems associated with transitioning of systems from second generation to third generation by providing a flexible data rate transmission in a telecommunications system that allows the gradual transitioning of systems by providing a flexible bandwidth that can be gradually increased during the transitioning.